The present invention relates in general to telephone systems, and, more particularly, to an electronic private automatic branch exchange and the arrangement incorporated therewith for effecting busy tests.
In our copending application Ser. No. 431,928, filed Jan. 9, 1974, entitled Matrix-PBX, assigned to the assignee of the present application, there is described an electronic private automatic branch exchange which is built around a space-divided rectangular solid-state switching matrix. One side of the switching matrix provides line appearances which are connected to line circuits, tone receivers, senders and operator loops. The other coordinate side of the matrix provides junctor appearances for connection with an attendant junctor, local junctors and trunk junctors. The solid-state switching matrix is a single stage matrix providing direct connections between line appearances and junctor appearances by the closing of a single cross point. The connections between lines within the system are effected simply by the interconnection of a pair of cross points associated with the respective lines and a selected junctor.
In the system there is included a clock circuit which imposes upon the system a rigid time sequence for operation of the various component elements in accordance with predetermined programs. The clock system generates distinct junctor time slots, each junctor time slot being subdivided into junctor slots for individual control functions. The system steps continuously in its scan of all junctors during a recurring time frame. In each junctor time, any calling or called line circuit associated with the junctor is addressed to obtain therefrom, on a common bus, signals which indicate the condition of the line from which the status of the line and request for service can be decoded. At the end of each complete scan of the junctors, the line scanner addresses a single one of the line circuits, to determine whether a request for service is present, the line scanner stepping to the next line circuit at the end of each complete scan of the junctors. By this means, the line scanner steps through all line appearances and thereby continuously scans the line circuits, while all of the junctors are being scanned in between each addressing of the respective line circuits.
Based upon the line and junctor scans, the system operates to allot a local junctor to any line going off-hook and a status circuit maintains a record of the state of that line circuit in a memory time associated with the particular junctor. In accordance with the state of the call, the common control circuits step the status of the particular line circuit from one program to the next until a communication connection is established from the line circuit through the switching matrix either to another line, the operator, or an outgoing trunk.
In the normal sequence of operations, when a subscriber makes a call, in order to determine whether or not it is possible for the subscriber to be connected with the desired called party, a busy test is carried out.
In conventional systems, busy test is carried out by a search for a free mark lead in order to get a path. If no free mark lead can be found, busy tone is applied to the line circuit of the subscriber. In this process, the system matrix is normally employed in conducting the search.
In the above-referred to electronic private automatic branch exchange disclosed in copending application Ser. No. 431,928, the switching matrix is employed only for providing audio between the talking parties. In the process of carrying out call connecting operations, memory circuitry within the common control stores the necessary information with respect to the lines associated with the system and the status of each call being processed. Associated with each call is a memory or storage area and since a junctor is allotted for each call, the memory will be associated with the junctor. The memory will contain all of the necessary information, e.g., the called and calling numbers, and other pertinent information, such as the status of the CB relay contact, which indicates whether a line is on-hook or off-hook.
If it becomes necessary to change junctors during the course of establishment of the communication connection, such as in the case of an outgoing trunk call, where switching between a local junctor and a trunk junctor is necessary once the destination of the call is determined, then the other junctors are either randomly addressed for information change or a temporary memory in the form of a hold register is used. A junctor associated with a communication connection through the switching matrix remains in connection with the line circuit or circuits involved throughout the call and, at the end of the call, the junctor memory is cleared and the particular junctor is released.